Cryogenic flex cable connector

ABSTRACT

A new electrical connector system employs two printed circuit board type connector components (1 &amp; 3), each containing a plurality of plated-on metal lines (11 &amp; 13) running along the board in parallel, and a replaceable contact element (9), termed an interposer, that is sandwiched between those printed circuit boards. The interposer contains malleable electrical contacts (15 &amp; 17) on each side that extend electrically from one surface to the other and serves as a contact bridge for those connector contacts. The interposer is formed as a thin insulator strip and metal bumps on the top and bottom sides of the strip serve as the electrical contacts. Bridging connection for metal bumps on the top side with associated bumps on the bottom side is served with plated-through holes (16). The metal bumps are of Indium and Tin that is covered by a flash of Gold. Ancillary to the described connector, a new method for constructing an electrical connector is also presented.

This is a division of application Ser. No. 08/886,312, filed Jul. 1,1997.

FIELD OF THE INVENTION

This invention relates to electrical connectors, and, more particularly,to a flex cable connector system useful in cryogenic environments.

BACKGROUND

A number of companies, such as IBM and Packard-Hughes, produce highdensity flexible electrical cables, often referred to as "flex cable",for use in routing electronic data. These cables have the capability ofrouting data at high rates of up to 10 Gigabits per second. The cable isrelatively flat and resembles a thick stiff belt in appearance and itmay be bent around corners or wrapped, much like an ordinary leatherbelt. The flex cables pack a large number of separate insulatedelectrical lines within the flex cable's limited width, typically at aline density greater than eighty signal lines per inch. In itsconstruction, the flex cables employ a polyimide film, a strong flexibleplastic insulating material, as the dielectric substrate and outerinsulating wrap. The electrical lines are lithographically defined andare formed of a very thin and narrow metal strips upon the dielecticsubstrate and a covering layer of the same material is laminated to thatsubstrate covering the metal strips.

The foregoing flex cables may be designed to have low thermalconductivity, a characteristic which makes the cable ideal for cryogenicapplications. The cables can be used to connect cryogenic electronicsapparatus, superconducting electronic apparatus, cryo-CMOS circuits andcooled GaAs amplifiers, which a cryogenic refrigeration system maintainsat very low cryogenic temperatures during operation, to other externalelectronic components and circuits that are maintained at roomtemperature. Since the cable doesn't conduct significant external heatto the cryogenic apparatus, the cable does not create an undue heat loadon the cryogenic refrigeration system.

For expeditious cable connection and/or disconnection, electricalconnectors are employed with the cable. Respective lengths of cable arewired into respective electrical connectors and the lengths of cable areinterconnected by connecting the two mating electrical connectorstogether, as is conventional practice in the electronic field.

As is elementary, an electrical connector contains a sufficient numberof spaced electrical contacts, enabling each connector contact to beelectrically connected to a respective electrical lead in the cable. Acable to cable connection is made by mating two connectors or connectorportions, as variously termed, together, to form the connector system orconnection, and bridging the multiple electrical paths from one cable toanother through the connector contacts. To avoid possible confusion inthis description, it is appropriate to remind the reader that eachmating half or portion of a connector system is customarily alsoreferred to as a connector. When reference is made to connector, thus,the reader should be certain to understand the context in which thereference is made to understand whether reference is being made to aconnector portion or to the connector system.

The electrical connectors used by the afore-recited flex cablemanufacturers to connect flex cables together or to connect flex cablesto rigid printed circuit boards employ a "pressed contact" arrangement.The pressed contact arrangement is not the typical male-female prong andsocket contact arrangement found in conventional electrical connectors,in which a prong contact frictionally engages within a socket contact.Instead, in the situation in which two flex cables are to be joined, twosubstantially identical relatively planar thick rigid printed circuitboards, containing the requisite number of electrical lines formedslightly protruding above the circuit board's planar surface serve asthe mating connectors. And, in the situation in which a flex cable is tobe connected to a printed circuit board, the flex cable and the rigidprinted circuit board, containing the requisite number of electricallines protruding slightly above the planar surface of the circuit board,serve as the mating connectors.

As those skilled in the art appreciate, for a cable to cable connection,the electrical lines plated upon each of those circuit boards is alignedand soldered or otherwise joined to corresponding electrical leads at anend of an associated flex cable, typically by conventional solderingtechniques. One of the circuit boards in the connector is invertedrelative to the other and, with the electrical lines on the circuitboards aligned, the boards are pressed into engagement to place therespective lines in electrical contact and form the electricalconnection. To complete the connector, a mechanical fastening system,including alignment pins and a pressure pad of elastomeric materialclamps the mated connector portions together and maintains therespective parallel conductors in contact under a positive pressure orforce. The surface of pressure pad contains a series of minute elasticrubber-like bumps or fingers to press against the top of one of thecircuit boards, providing, thus, the pressed contact arrangement.

Although the foregoing connector design serves well at room temperature,at cryogenic temperature the connector, and, particularly, theconnector's elastomeric pad, often fails to function properly. Atcryogenic temperature, the elastomeric material forming the pressurepads becomes brittle and loses its ability to maintain adequate pressureon the circuit boards. As example, where the connector connects eightyor so electrical lines in a high density flex cable, should any one orthose electrical lines fail to connect through the connector to anassociated line, the connector is deemed to have failed. The loss of anyelectrical path through the connector cannot be tolerated. Thus,although available flex cable is ideal for application in cryogenicdevices, presently available connectors for those flex cables areunsuited for use at those very low temperatures.

When the foregoing connector fails, it must be replaced. To do sorequires the cable to be disconnected from the old structure andreattached and soldered to the replacement or requires a new cable to beattached and soldered. In addition to requiring new connectors, thatprocedure also requires considerable time and expense.

An object of the present invention, therefore, is to provide a newconnector system for flex cable that functions at cryogenictemperatures.

A further object of the invention is to provide a connector system thatmay be easily repaired or reconstructed without requiring cable rewiringanew, should the connector system fail.

Another object of the invention is to provide a connector system thatadapts flex cables to cryogenic device application and may be used incryogenic systems.

And a still further object of the invention is to provide a positivepressure contact type connector that does not incorporate elastomericmaterial or any other material that becomes brittle or disfunctional atcryogenic temperatures.

SUMMARY OF THE INVENTION

In accordance with the foregoing objects, an embodiment of the presentinvention employs two printed circuit board type connector components orconnectors, each containing a plurality of plated-on metal lines runningalong the board in parallel, that serve as the connector contacts, and areplaceable contact bridging element, termed an interposer, that issandwiched between those two circuit boards and engages those circuitboards under a positive force. The interposer contains electricalcontacts on each side that extend electrically from one surface to theother and serves as a contact bridge for those connector contacts. Eachcontact is placed in engagement with a corresponding line on the twocircuit boards to electrically connect the lines.

In accordance with an aspect of the invention the interposer is formedas a thin strip of electrically insulative material and the interposer'scontacts are small malleable metal bumps on the top and bottom sides ofthe strip. In accordance with a still further aspect to the inventionelectrical bridging between a metal bump on the top side and anassociated bump on the bottom side is accomplished with plated-throughholes. And, in accordance with a still further aspect, the metal bumpsare formed of an alloy of Indium and Tin and, preferably, the Indium-Tinalloy is covered by a flash layer of Gold.

Advantageously, any thermally induced constriction that occurs throughlowering of temperature to cryogenic levels enhances the electricalcontact. Should the pressure become too great, by design, the metalbumps would yield, but maintain the electrical connection nonetheless.When the connector is released upon completion of one cryogenicoperation and readied for reuse in a subsequent cryogenic operation, anydistortion of the metal bumps resulting from the former operation asmight cause failure of the connector system, is easily remedied bydiscarding the suspect interposer and replacing that interposer with anew one. No rewiring of cables is required. The foregoing new connectormay be supplied with ample supplies of replaceable interposer elements,assuring the integrity of the electrical cabling for each and everycryogenic operation.

The foregoing and additional objects and advantages of the inventiontogether with the structure characteristic thereof, which was onlybriefly summarized in the foregoing passages, becomes more apparent tothose skilled in the art upon reading the detailed description of apreferred embodiment, which follows in this specification, takentogether with the illustration thereof presented in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a pictorial view of the an embodiment of the connector priorto final assembly;

FIG. 2 is a pictorial view of the embodiment as assembled with thecomponents sandwiched together to form the connector;

FIG. 3 is a partial side view of the embodiment of FIG. 2 drawn to alarger scale;

FIG. 4 is a top plan view of once of the interposer component of theconnector;

FIG. 5 illustrates the connector component of FIG. 4 in side view;

FIG. 6 is an enlarged partial side view of a portion of the viewpresented in FIG. 5;

FIG. 7 illustrates the interposer element in top plan view drawn to alarger scale than the companion component of FIG. 4, with the bottomview of the interposer element being a mirror image of the top;

FIG. 8 is an enlarged section view of the interposer element taken alongthe lines 8--8 in FIG. 7;

FIGS. 9, 10 and 11 illustrate the assembly of a cable to cable connectorinto a completed connector and the clamping structure;

FIG. 12 illustrates the process for forming the interposer component ofFIG. 4; and

FIG. 13 illustrates a cable to circuit board embodiment of the connectorsystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is made to FIGS. 1, 2 and 3 which pictorially illustrate theprincipal elements of the disclosed electrical connector or, asvariously termed, electrical connector system. Referring first to FIG.1, the connector, shown partially dissassembled, contains a pair ofprinted circuit boards 1 and 3 and a small flat strip 9, here called an"interposer". The connector is illustrated as part of a cable to cableconnection system, and printed circuit boards are attached to respectiveflexible cables 5 and 7. As shown in partially assembled form in FIG. 2,the printed circuit boards 1 and 3 are placed one atop the other, withprinted circuit board 3 inverted, and interposer 9 is sandwiched inbetween.

Referring again to FIG. 1, circuit board 1 contains multiple parallelmetal electrical traces or lines 11 spaced apart on the board's topsurface. As is conventional with circuit boards, the metal lines areplated or otherwise bonded to the circuit board surface using knownprocesses and suitably comprise Gold plated Copper. Electrical lines 11are electrically connected to corresponding electrical leads in theassociated cable 5 at the rear end of the circuit board. Likewisecircuit board 3 contains identically spaced parallel metal electricallines 13 on its top surface and is similarly electrically connected tolike electrical leads in cable 7 associated therewith. Interposer 9contains like laterally spaced malleable metal bumps 15 on its topsurface, and, not visible in this figure, also contains correspondingspaced malleable metal bumps on its underside surface as well. Eachmetal bump on the top surface is aligned with a corresponding one of themetal bumps on the bottom surface of the interposer, which is visible inFIG. 3, next described.

Reference is made to FIG. 3, which provides a side view of a portion ofFIG. 2, drawn to a larger scale, to better illustrate, the metal bumps15 and 17 on the respective upper and lower surfaces of strip 9. Themetal bumps are attached to and supported by a thin strip 10 of flexibleelectrical insulating material, preferably a polyimide film, andprotrude slightly from the surface of the film. Metal bump 15 on the topsurface, only one of which is shown, is in contact with an overlyingelectrical line 15; and the metal bump 17 on the underside, verticallyaligned with bump 15, is in contact with an underlying one of theelectrical lines 11 on circuit board 1.

Film strip 10 contains a number of cylindrical passages or holes,represented by dash lines, extending through the strip's thickness thatcorrespond in number to the number of bumps on the top surface. Each ofthose holes is in alignment with a respective pair of top and bottompositioned metal bumps, such as that illustrated for metal bumps 15 and17. Further, the holes are "Plated-through" with metal, such as copper.That is, the walls defining the hole are metalized or plated with metal,and are formed by an entirely conventional process long known in theprinted circuit board art. The plated-thru metal cylinder so formedelectrically connects at each end to a respective metal bump, 15 and 17.It is appreciated, thus, that when assembled together as illustrated inFIG. 3, the completed connector serves to provide an electrical paththrough the connector for each of the electrical lines in the associatedcables.

Metal bumps 15 and 17 are formed of an alloy of Indium and Tin, which,as later herein discussed, has the desirable characteristic of beingmalleable. A preferred composition to that alloy is 52% Indium and 48%Tin. Because Indium-Tin could form dendrites allowing physicallycontacting Indium-Tin bumps to possibly "grow" together, which wouldmake dissassembly of the connector difficult, the Indium-Tin bumps arepreferably flash plated with Gold. Because Gold is non-corrosive anddoes not become "sticky" the interposer is more easily detached from theassociated printed circuit boards of the connector and allows theconnector to be more easily disassembled.

The foregoing pictorial illustrations afford introduction to the newconnector structure and illustrate its general design principals using alimited number of electrical contacts. A more complete illustration isprovided in the succeeding drawing figures, which are next considered.For convenience in the following figures, the same number designation isgiven to an element that corresponds to an element previously identifiedin FIGS. 1-3.

A more typical cable or connector structure is presented in top planview in FIG. 4 and in side view in FIG. 5, to which reference is made.Cable 5, partially illustrated, is a typical high density cable. Itcontains 128 parallel electrical lines or, as variously termed, signaltraces 12 that are of three mils line width on an eight mil pitchlaminated between thin layers of polyimide. The corresponding relativelyrigid circuit board 1 contains a like number of lines or traces 11 oflike width and pitch. To give a better feel to the small sizes possiblefor the connector invention, it is noted that in a practical embodimentof the connector of FIG. 4, the width of the connector, along theX-direction in the figure, is about 1.33 inches, and the length of theconnector, along the Y-direction in the figure is about 0.7 inches.

The cable is attached to the printed circuit board and the individualleads are connected to associated electrical leads on that circuitboard. The latter is accomplished by conventional soldering technique.As illustrated in the enlarged partial side view of FIG. 6, which istaken along line 6--6 in FIG. 5, a short length of the coveringpolyimide layer 5b is removed from an end of the cable to expose theelectrical leads 12, the upper covering layer 5a remaining unchanged,the exposed electrical leads 12 are aligned with the correspondingelectrical leads 11 on the circuit board; and solder is applied tomechanically and electrically join leads 12 and lines 11 together.

Returning to FIGS. 4 and 5, cable 5 and circuit board 1 are clampedtogether between bars 19 and 21, located at the rear end of theconnector. Narrow flat rectangular metal bar 19 abutting the upper cablesurface serves as one clamp and the complementary more thin flat bar 21located abutting the opposite or bottom surface of the circuit boardserves as the complementary clamp bar. Each such bar extends across thewidth of the printed circuit board and covers a fraction of the circuitboard's length. Bar 19 contains a clearance hole at each end and bar 21contains tapped holes, aligned with the holes in the complementary clampbar 19. Bolts 20 and 23 fit through the respective passages in bar 19,underlying aligned passages in printed circuit board 1 and are threadedinto the respective tapped holes in bar 21. The clamp provides a strainrelief for cable 5, preventing any pulling force on cable from reachingthe soldered lead connections with the circuit board and breaking one ormore electrical connections.

Relatively rigid printed circuit board 1 also contains at least twospaced alignment holes 25 and 27, which are used to properly positionthe interposer 9, as later herein described.

Temporary reference is made to FIG. 9 which shows both connector 1 andconnector 3 in side view. The remaining connector 3 to the connectorsystem is of the same structure already illustrated and described inrespect to connector 1, which need not be repeated. Connector 3 is alsoconnected to its associated electrical cable 7 in the same manner asthat used for connector 1, which is also not here repeated.

The interposer 9 for this embodiment is illustrated in top plan view inFIG. 7, to which reference is made, and is drawn to a slightly largerscale than FIG. 4. The element is formed of a very thin rectangularlayer or strip 10 of plastic film material that is electricallynon-conductive, suitably a 2 mil layer of Polyimide. The strip isrelatively flexible relative to the stiffness of the circuit boards 1and 3, which are relatively rigid. The strip is sufficient in length toextend across the width of those printed circuit boards.

Malleable metal bumps 15, which serve as electrical contacts, protrudefrom the upper surface of the film. Those bumps are arranged in twospaced parallel rows, spaced along the strip's length front to rear, theY-direction in the figure, with one row vertically above the other asillustrated in the figure. A like arrangement is used for the bumps onthe opposite side of the strip, wherein each bump on the upper surfacevertically overlies a corresponding metal bump protruding from thestrip's bottom surface. Arranging the metal bumps on a side in staggeredrows allows a greater clearance space between metal bumps. That greaterclearance minimizes the chance that a solder splash or metal dendritewill bridge between adjacent bumps to create a cross-circuit betweenconductors in the connector.

The spacing between bumps along the width or the X-direction in thefigure is the same spacing used with the conductors in the connector 1and in connector 3. The spacing between adjacent bumps in the same rowis twice the foregoing distance. The contacts may be arranged in threeor more rows in other embodiments should an even greater clearance spacebe desired or if it is desired to reduce the width of the connector.

Pin alignment holes 26 and 28 on the left and right ends of the strip 9form passages through the thickness of the strip. Those passages arespaced apart by the same distance as pin alignment holes 25 and 27 inthe connector illustrated in FIG. 4. The pin alignment holes permitbumps 15, and the corresponding metal bumps on the opposite side of thestrip, to be properly aligned with the associated conductors on theprinted circuit boards in connector components 1 and 3, when theconnector is assembled together, as later herein described.

As illustrated in enlarged scale in the side section view of FIG. 8,which is taken along the line 8--8 in FIG. 6, film 10 containscylindrical holes 14. Those holes each contain a metal cylinder 16,extending from the top to the bottom, which is formed on the walls ofthe respective hole. The metal cylinder serves to electricallyinterconnect coaxially aligned bumps 15 and 17. In this embodiment thebumps 15 formed are semi-spheroidal in shape, are connected atop thebump pads 18, an annular metal plated area surrounding the entrances tothe holes, and to the metal cylinder 16. In a practical embodiment, thebumps are five mils in diameter and overlie and underlie, respectively,through-hole passages in the strip that are of two mils in diameter; andthe spacing of adjacent bumps in a row is 0.016 inches and the rowspacing is 0.028 inches.

One process for fabricating the bumps on the interposer element,illustrated in FIG. 12, is considered. First, both sides of thepolyimide film 10 are coated with copper as at 40. This is accomplishedeither by sputtering a seed layer of copper on both sides of the filmand then electroplating the seed layer to build up a thicker layer ofcopper or by laminating copper foils directly to the film layer'ssurfaces with adhesive. Both techniques are known in the printed circuitboard art. The holes 14 and the alignment holes 26 and 28 are thendrilled through the plated layer with appropriate drilling apparatus, asat 41, which in the case of the minute holes employed in the example, isaccomplished by laser machining techniques.

The holes are then plated through with a metal, as at 42, usingconventional plate through plating processes well known in the printedcircuit board art. The cylindrical walls defining the drilled holes arenow covered with a metal, a metal cylinder, that forms an electricallyconductive path between the top and bottom surfaces of the plated stripand that joins with the copper plating on the top and bottom surfaces.

The bump pads are then defined and formed, as at 43, by usingphotolithographic technicues. In this technicue, a photoresist materialis applied to the film, and the photoresist is exposed in those areaswhich are to remain copper covered. The unexposed resist is removed,leaving only the bump pads and metalized through-holes covered with anacid resistant coating. Then all the copper, except for the bump padsand the plated through holes, which remain covered with an etchresistant layer, is etched away with a suitable etchant. The film layeris then washed to remove all traces of the etchant and the film layer isready for further operation to form the metal bumps.

Both sides of the film layer subassembly are then brushed with a fluxcontaining a part activating flux and part leveling flux, as at 44. Thefilm is then dipped into a molten solder bath containing a eutecticindium/tin solder heated to approximately 130 degrees Centigrade, as atstep 45. The film layer subassembly is then withdrawn from the bath andthe solder which adhered to the exposed copper, such as on the bump padsand over and about the smaller connector contact holes, solidifies. Inthat action the solder wets the bump pads and, due to the inherentsurface tension of the molten metal, it also forms a hemispherical bumpover the ends of the plated through holes.

The physical phenomena that occurs to form the bump is much likeobserved by children in which a small magnifying lens is formed withwater that is produced simply by dipping a small eyelet into water andwithdrawing it. Due to its inherent surface tension, if the eyeletdiameter is small enough, the water sticks to the eyelet and forms aconvex shape meniscus, a hemispherical shape, defining the magnifyinglens.

After the solder solidifies, the Indium-Tin bumps are coated with a thinlayer of electroless gold plating, as at 46. For economy of scale inmanufacturing the interposer, optionally, a single long strip containingmany such interposers end to end may be processed in the foregoingmanner in one continuous strip and, upon completion of that treatment,the strip is then cut into individual interposer strips, as at 47.

The described connector components are assembled together to complete aconnection as illustrated in the side assembly views presented in FIGS.9, 10 and 11 to which reference is made. As shown in FIG. 9, theconnector components 1 and 3, previously assembled to respective cables5 and 7, are arranged one over the other and the interposer 9 componentis inserted in between and laid down atop the conductors 11 with itsalignment holes overlying the corresponding alignment holes in connector5. As shown in FIG. 10, connector component 3 is then brought down intocontact with the interposer with its alignment holes aligned with theinterposer's alignment holes. In that way the electrical contacts oninterposer 9 are brought into contact with the metal traces on theadjoining connectors, and forms a bridging path between the trace on thecircuit board of connector 1 to an associated trace on the complementaryboard of connector 3.

To hold the elements in the foregoing relationship and complete theconnector a fastening structure is used that clamps the elementstogether. That clamping force also maintains the contacts under apositive pressure or force. Such a clamp is formed with upper and lowerclamping bars 31 and 33, and bolts 32 and nuts 34. Each of the clampbars is of a straight narrow rectangular box shape, as illustrated inend view in the figure, and is of a length that extends across the widthof the connector.

Upper and lower clamp bars 31 and 33 are placed in contact with therespective rear surfaces of the circuit boards of connectors 3 and 1,respectively, with their alignment holes in alignment with thecorresponding alignment holes in each of those circuit boards. Bolts 32,only one of which is illustrated, are inserted through the holes and arefastened in place with nuts 34, only one of which is illustrated. Thefastened bolts thereby maintain the components in the illustratedsandwich relationship. The bolts and nuts exert force on the adjacentclamping bars, and the bars distribute such clamping force across theentire underlying surfaces.

Preferably a portion of the bolt's shank leading from the bolt head isleft unthreaded, leaving a smooth cylindrical surface of a diameter justsmall enough to clear the cylindrical walls of the alignment holes inthe circuit boards and interposer. The foregoing structural featureallows the fastening bolt to serve also as an alignment pin that ensuresthat the parts remain in the desired alignment through final assembly.

The connector remains effective at cryogenic temperatures. When cooledall of the metal parts contract with temperature as would enhance firmelectrical contact between the elements. By intent, the interposer isthe one element which is intended to give way under such pressures. TheIndium-Tin bumps on the interposer yield and flare against the addedreverse thermally induced force. When the equipment is to be disengagedand removed, the cryogenic temperatures are removed and the unit isbrought back to room temperature. The connector is easily dissassembledby unfastening the nut, removing the clamping structure and detachingthe component elements, in essentially a reverse procedure to thatearlier described in connection with assembly in FIGS. 8 through 10.Because the bumps on the interposer were distorted, as described above,the interposer is discarded. When the connection is again to be made, anew interposer is substituted, thereby making the connector as good asnew.

As earlier described, the interposer is a simple structural element andmay be constructed almost entirely by automated fabrication equipment,making it a relatively inexpensive component. Instead of requiringsubstitution of a new connector and the attendant labor of rewiring anew connector to the cable, great savings in both time and effort areachieved by merely inserting a new self-contained component into place.

The foregoing illustrated a cable to cable connection. It is appreciatedthat the foregoing connector also may be used for a cable to circuitboard connection, such as is illustrated in side view in FIG. 13. Inthis embodiment, the left connector 3' is of the same constructionillustrated in FIGS. 4, 5 and 6. However, the second connector portion1' is the edge of the equipment's printed circuit board, whichsubstitutes for the second connector portion. That equipment circuitboard contains appropriate alignment holes. And the interposer 9' andthe associated clamping device 31', 32', 33' and 34', is the same as inthe connector structure earlier described.

Many additional variations are possible. The metal bumps used in theforegoing embodiment were formed of an Indium-Tin, a metal thatpossesses the requisite degree of malleability. However other malleablemetals may be substituted for Indium-Tin in the combination thatpossesses a Moh's hardness rating within the range of 1.2, the hardnessrating of pure Indium, to 3.0, the hardness rating of Gold.

Further, the foregoing embodiment used metal bumps that were formed of aspheroidal shape and were formed in a particular process. However, asthose skilled in the art understand from reading this specification, theconnector invention is not so limited as other shapes may be substitutedand the metal protrusions may be formed by other processes. So long asthe metal selected is malleable and compresses, the bump should properlyfunction.

Additionally, the preferred embodiment has been illustrated inconnection with connectors having very fine traces and using very minutesized metal bumps as exist for a particular cryogenic application knownto the inventors, which gave rise to the problem solved by the disclosedconnector. However, it is recognized that the disclosed connectorstructure may be adapted to larger or smaller size metal traces in thesame or for different applications and may find application in otherfields for related reasons.

Lastly, the metal bumps for the embodiment were fabricated using aspecified dipping process. Other fabrication processes may be choseninstead. As example, the metal bumps may be separately formed and may beapplied one by one by automated equipment to the ends of the holes andaffixed by an electrically conductive epoxy. Alternatively they may beso positioned and then electrically welded. Notwithstanding, the dippingprocess is preferred for small diameter bumps, of five mils or less indiameter.

It is believed that the foregoing description of the preferredembodiments of the invention is sufficient in detail to enable oneskilled in the art to make and use the invention. However, it isexpressly understood that the detail of the elements presented for theforegoing purpose is not intended to limit the scope of the invention,in as much as equivalents to those elements and other modificationsthereof, all of which come within the scope of the invention, willbecome apparent to those skilled in the art upon reading thisspecification. Thus the invention is to be broadly construed within thefull scope of the appended claims.

What is claimed is:
 1. The method of constructing an electricalconnector which includes the steps of:forming a plurality of holesthrough a thin elongate strip of electrical insulating material; platingthe walls of said holes with an electrically conductive metal to formplated-through holes; and dipping said thin elongate strip in a bathcontaining molten metal and withdrawing therefrom to form bumps of metalon each end of said plated-through holes.
 2. The method as defined inclaim 1, wherein said metal comprises an Indium-Tin alloy and furthercomprising the additional step of plating said bumps with a metalcomprising Gold.
 3. The method defined in claim 1, wherein saidelectrical insulating material comprises polyimide.
 4. The method asdefined in claim 1, further comprising the step of cutting said thinelongate strip containing said metal bumps on each end of saidplated-through holes into a plurality of shorter thin elongate strips.5. The method as defined in claim 1, further comprising the step,following said step of forming a plurality of holes, of plating anelectrically conductive metal annuluses on the upper and lower surfacesof said strip surrounding the respective entrance and exit ends of eachof said plurality of holes to define a plurality of bump pads.